The tie rods are ball joints and if they become jammed up with dirt, I could see there being a rough spot. There is really only one main bushing on the gear housing, but I don't see how that would create a rough spot. I don't see how a bushing would do that anywhere. It would think a rough spot would need to be where two pieces of metal rub together which either means a ball joint, bearing, or gear itself. Since gears and bearings are sealed, rain should have no affect, so that kind of points to the tie rods. They are also the easiest thing to swap out. Just put the car, take the wheels off. take the cotter pin out, take the nut off, then pop the tie rods out with your weapon of choice. I use the small pitman arm puller from autozone (27022). Then just move the ball joint around with your hand to see if it moves smoothly with no excessively loose or tight spots.